1. Field of the Invention
The present invention is directed to a method and apparatus for constraining an edge of a signature in a printing press. In particular, the present invention is directed to a method and apparatus which prevents dog-earring in the open edge of folded signatures which are created in a printing press apparatus, as the folded signatures are transferred between components in the printing press.
2. Description of the Prior Art
In printing presses which print on a continuous web of material, such as, for example, an offset printing press, it is known to cut the printed web into signatures and thereafter to fold the signatures into what are sometimes called books. A signature is a cut section of a web of material in a printing press, whereas a book is a signature that has been folded in one or more places. FIG. 6 is a representation of a signature S which has been folded into a book, and which has a closed or folded edge F and open edges O. Folded signatures S in the form of books are collated and bound together to form a printed product, such as a magazine.
In modem printing presses, it is necessary to cut the web into signatures and fold the signatures into books at very high rates of speed. Alternatively, the web can be folded first, and then the folded web cut into books. Various mechanisms are known for accurately cutting a printed web into signatures of relatively uniform size, and thereafter folding the signatures into books of relatively uniform size, all at high rates of speed. Various mechanisms are also known for folding a web, and thereafter accurately cutting the printed and folded web into books of relatively uniform size, also at high rates of speed.
In known printing press apparatuses, folded signatures in the form of books emerge from cutting and folding mechanisms via a tape drive. FIG. 4 is a side, cross-sectional view, and FIG. 5 a partial perspective view, of a prior art printing press in which a folded signature S emerges in a direction D from a cutting and folding mechanism M via a tape drive T The tape drive T includes one or more tapes or belts B, moving continuously in a direction d, which engage at least one side of the folded signature or book S and conveys it in a linear direction. Usually, the tape drive T will include a tape or belt B which overlaps the open edges O of the folded signature or book S and is of sufficient width so that variations in the folded width W of the folded signature or book S will result in the open edges O always projecting under the tape or belt B. That arrangement therefore ensures that the open edges O are held down by the tape or belt B, preventing dog-earring of the comers C of the open edges O.
The tape drive T in the prior art apparatus of FIG. 4 transfers the folded signature or book S to transfer device 101. The transfer device 101 may rotate in a direction R1, and include some mechanism to secure a folded signature or book S to its outer surface. The signature S shown in FIG. 4 is transferred by tape drive T to the outer circumference of the transfer device 101, and after traversing a gap G in an unconstrained manner, the leading edge l of signature S is gripped by a leading edge gripping mechanism 4, 5 which projects from the periphery of a deceleration drum 6 (the periphery of which is indicated in dashed lines) rotating in a direction R2.
One problem with the known folded signature or book transfer mechanism as shown in FIGS. 4 and 5 is that the comers C of the open edge O of the folded signatures or books S have a tendency to dog-ear during transfer from the tape drive T to the deceleration drum 6, from the tape drive T to the transfer device 101, or from the transfer device 101 to any other transfer mechanism, such as a downstream deceleration drum. Dog-earring is a condition where a corner C' or edge of a flexible sheet or signature bends or folds over in an undesirable manner, as shown in dashed lines in FIG. 6. One solution which has been used to reduce the occurrence of dog-earring of the folded signatures or books S during the transfer steps is to electrostatically charge the folded signatures or books S. In one prior art arrangement, the web passes over a folder and is folded in one or more places. The folded web is then electrostatically charged by producing a voltage across the folded web, and then the charged web is cut into folded signatures or books S. As a result, the charge on the folded signature or book S results in the sides of the folded signature or book S being electrostatically attracted, and adhered, to one another. This adhering of the two charged sides results in the open edges O adhering to one another, thereby increasing the thickness of the open edge O presented to the transfer mechanisms, and reducing the chance that a corner will be unrestrained and become dog-eared.